Essentially, two methods have gained acceptance for parallel control of tap transformers.
The first method, i.e. so-called synchronous control, can be carried out only on transformers of the same voltage, power, short-circuit voltage, tap voltage and tap number. In this method all transformers of the parallel group have to be in the same operational setting. This method, also known as ‘master-follower method’, is otherwise the sole method for parallel control of single-phase transformers in a three-phase alternating current group.
The second method, which is disclosed in European Patent Application EP 2 389 723 A2, operates according to the so-called circuit reactive current method. This form of control of parallel operation is principally used in parallel operation of transformers with different tap numbers and different tap voltages. The method for parallel control makes auxiliary apparatus—apart from voltage regulators—no longer necessary, so that it is possible to dispense with current-conducting cross-connections between measurement transducers of transformers running in parallel. In addition, it is ensured that there is no influence from transformers, which are taken out of parallel operation, on the parallel operation of the remaining transformers and no changes are required in the settings of the control sensors. All measurements detected by the current or voltage transducers are now derived mathematically. It is thereby possible to provide all electrical parameters necessary for control of parallel operating transformers with very simple transducer circuits. The wiring between the measuring transducers and the respective control sensors is thus reduced to a minimum so that even in the case of parallel control of transformers in each instance only one current transducer and one voltage transducer are needed for each transformer. The control sensors of the individual transformers are all connected together by a CAN bus. The measurements are thus interchanged between the individual control sensors. If interruption of the CAN bus occurs, the parallel circuit shifts into emergency operation or in the worst case the parallel circuit takes from the mains.
International Patent Application WO 93/17480 A1 discloses an arrangement for voltage regulation in parallel operation of transformers. Each transformer comprises a tap changer. This is connected with a microprocessor-controlled digital regulator (control sensor). The me regulates the tap voltage. In addition, each digital regulator has an interface by which the digital regulators are connected to form a ring and the digital regulators exchange the measurement and controlling values with one another by this ring. They regulate the compensating current, which flows between the transformers, to a minimum by these values. The tap changers of the individual transformers are automatically led into an operational setting in which the circuit reactive current between the transformers reaches a minimum.
European Patent Application EP 0 482 361 [U.S. Pat. No. 5,210,443] discloses a method for parallel operation regulation of tapped transformers acting in any switch combinations on a double-busbar system. In that case, associated with each tapped transformer is a separate regulator which acts on the associated motor drive. The me in turn actuates the associated tap changer. In the method, initially the instantaneous transformer configuration is detected in a parallel control device. Subsequently, the instantaneous amplitude and phase values of voltage and current of all tapped transformers are communicated to this device, whereupon the part load current and the circuit reactive current are detected for each tapped transformer and a disturbance variable for the respective control sensor is formed therefrom.
German Patent Specification DE 40 04 671 C1 discloses a method for automatic computer-assisted recognition of plant configuration in the case of a plurality of tapped transformers which are connected in parallel as desired to different busbars and which can have different tap numbers and tapped voltages, as well as the regulation thereof. In that case, regulators associated with each tapped transformer respectively cyclically detect the active and reactive components of current and voltage. The association between transformer and corresponding busbar is undertaken in a master regulator. Subsequently, for all busbars the circuit reactive currents of the transformers connected therewith are determined in succession. Finally, the corrected voltage values are fed back to the regulators as new target values.
German Patent Application DE 100 39 918 A1 discloses self-adapting power-factor regulation (PF regulation) for minimization of circuit currents arising in mains due to parallel connected three-phase alternating current transformers. The electromotive force (EMF) of the transformers is varied in dependence on a determined value which is obtained by comparison of the cosf at the feed point of each transformer (cosfactual) with a target cosf of the mains (cosftarget) wherein the cosftarget once correctly input at the time of setting the circuit current minimization is automatically adapted to the subsequent changes over time of the mains relationships.